There exists a requirement to screen containers for the presence or otherwise of illegal materials and devices. Often such containers are large (for example a 40 ft×8 ft×8ft ISO container) and may contain dense loads such as machinery. In order to provide a non-intrusive and fast inspection process, a number of imaging systems have been developed that use high energy X-ray sources to penetrate through even large containers.
Typically these systems combine a high energy X-ray linear accelerator source with a series of X-ray detectors arranged in a linear fashion to create a one-dimensional inspection line through the object. By scanning the object through the X-ray beam at a controlled velocity, a set of one-dimensional projections are collected which can then be re-formatted into a two-dimensional image for subsequent operator review.
In all such known systems, the image collecting process is inherently undersampled according to the Nyquist sampling theorem which states that the data should be sampled at twice the rate of the highest bandwidth in the signal.
Also, X-ray scanners for cargo inspection typically comprise a high energy X-ray source (usually based on an X-ray linear accelerator) with a beam quality of 1 MeV to 9 MeV. The X-ray output from the X-ray linear accelerator is then collimated down to a narrow fan-beam of radiation which is shone through the item of cargo under inspection. A linear array of X-ray detector elements is then positioned opposite to the X-ray source such that it is irradiated by the fan-beam of radiation after attenuation of the X-ray beam by the object under inspection.